System for spatially adjustable excitation of leadless miniature marker

ABSTRACT

A system for generating a magnetic field for excitation of a leadless marker assembly. The system of at least one embodiment includes a source generator that generates a plurality of alternating electrical signals each having an independently adjustable phase. A plurality of excitation coils are configured to simultaneously receive a respective one of the alternating electrical signals at a selected phase to generate a magnetic field. The phase of the alternating electrical signal for each excitation coil is independently adjustable relative to the phase of the alternating electrical signal for the other excitation coils so as to adjust the magnetic field from the respective coil. The magnetic fields from the excitation coils combine to form a spatially adjustable excitation field for excitation of the remote leadless marker assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/044,056, titled SYSTEM FOR EXCITATION LEADLESS MINIATUREMARKER, filed Jan. 11, 2002, which is a continuation-in-part of U.S.patent application Ser. No. 10/027,675, titled SYSTEM FOR EXCITATION OFLEADLESS MINIATURE MARKER, filed Dec. 20, 2001.

TECHNICAL FIELD

This invention relates to systems for activating miniature markers, andmore particularly to systems for excitation of resonating miniaturemarker assemblies for use in locating the markers in three-dimensionalspace.

BACKGROUND OF THE INVENTION

Systems have been developed to activate and detect remote activatablemarker assemblies positioned, as an example, in or on a selected item orobject. The markers generate a signal used to detect the presence of themarker. Many of the activatable markers are hard-wired to a power sourceor other equipment external from the object. Other systems have beendeveloped that utilize resonating leadless markers, also referred to aswireless active markers, positionable at or near a selected target.These wireless active markers are typically activated or energized by aremote excitation source that generates a strong continuous excitationsignal. Accordingly, the markers generate a detectable marker signalthat must be distinguished from the strong continuous excitation signaland then analyzed in an effort to try to accurately determine thetarget's location. The process of distinguishing a weak marker signalfrom the strong continuous excitation signal, while maintainingsufficient accuracy and repeatability for determining the marker'slocation, has proven to be very difficult.

Other systems have provided detection of leadless markers to determine atwo-dimensional proximity (e.g., X, Y coordinates) to detectors for usewith game boards, surgical tag detection devices, and medical tubeplacement verification systems. In the case of the game boards, a uniquegame piece with a resonator of a predetermined frequency is moved acrossthe game board, and the X and Y location coordinates of the game piecerelative to the game board are displayed so the players can determinethe general location of the game piece on the game board. U.S. Pat. No.5,853,327 to Gilboa identifies that the X, Y coordinates, as a functionof amplitude or phase, may be determined experimentally for a given gameboard design. Additionally, Z distance away from the game board may bedetermined to a sufficient accuracy for game use by the strength of thesignal above the game board provided that the signal is not a strongfunction of the X and Y locations. U.S. Pat. No. 5,188,368 to Ryanprovides a system for determining in two dimensions which individualsquare of a chess board a particular chess piece is on during a chessgame. The system disclosed by Ryan does not determine the Z direction.

In the case of the surgical tag and detection device, U.S. Pat. No.6,026,818 to Blair discloses surgical devices, such as sponges, thathave activatable resonator tags thereon for proximity detection by ahand-held probe. The probe has a single loop interrogation ring providedthat can transmit an excitation signal to activate the resonator tag andthen be switched to a receiver mode. The probe is moved manually tochange the angular orientation of the interrogation ring, thereby movingthe resulting excitation field's orientation. The excitation field ismoved to a suitable orientation so as to excite resonator tags invarious spatial orientations. The interrogation ring can then be scannedover an area of a patient after surgery to determine if any surgicaldevices having the resonator tags have been left behind. Accordingly,the detection device of Blair is detecting the existence or proximity ofa surgical tag with the interrogation ring, rather than the actuallocation of the activatable tags.

In the case of the medical tube placement verification device, U.S. Pat.No. 5,325,873 to Hirschi et al. teaches a system that detects thegeneral position of an object within a body of tissue. The detectionsystem includes a three-axis resonant-circuit target attached to theobject and a separate hand-held detection probe having a pair ofparallel and coaxially aligned transmitter/sensing coils. Thetransmitter sensing coils generate a current that determines whether areturn signal strength of the target is great enough to be counted as avalid signal. The hand-held detection probe also has a pair of receivercoils positioned within each of the transmitter coils and connected in aseries-opposed fashion. The four receiver coils allow for the creationof a null circuit condition when the target is equidistant from each ofthe receiver coils. The detection probe also has a visual displaycoupled to the receiver coils and configured to indicate the direction(e.g., left/right/up/down) in which the probe should be moved to centerthe detection probe over the object, thereby achieving the null circuitcondition.

The systems of the above patents activate the markers with a pulsedexcitation signal generated by driving an untuned source coil witheither a unipolar polarity to produce a wide band impulse function or abipolar polarity to create a waveform that more closely matches thedesired resonant frequency of the marker. The required levels ofmagnetic excitation for the markers in the above patents are relativelylow such that the excitation energy in the source coil is substantiallyconsumed after each pulse due to the pulse circuitry resistive losses.The source coils are driven by linear amplifiers, and in one case bylinear amplifiers at both ends of the coil, and by a simple pulsenetwork that energizes the coil and extinguishes resistively. Theamplitude of the pulsed excitation signal required for theseapplications is relatively low since either the resonator circuit to belocated is of a large size, the volume in which the resonator must belocated is relatively small, or the accuracy requirements locating theresonator are quite low. Accordingly, the existing systems are notsuitable for use in many situations wherein highly accuratedeterminations of the marker's location in three-dimensional space isrequired. The existing systems may also not be suitable for use withefficient, high energy systems for energizing the marker assemblies soas to provide a sufficient marker signal for use in determining thelocation of the marker in three-dimensional space relative to remotesensors.

Other systems have been developed for tracking wireless tags for use asa tangible interface to interact with a computer. Such systems aredescribed in “Fast Multi-Axis Tracking of Magnetically Resonant PassiveTags: Methods and Applications,” Kai-yuh Hsiao, Massachusetts Instituteof Technology, 2001. As an example, one tracking system utilizedwireless markers tracked in three- dimensional space by a singletransmitter coil or by aligned multiple coils in a Hemholtzconfiguration. The multiple coils are continuously swept through a rangeof frequencies to activate the magnetically resonant passive tags, andthe transmitter coils are simultaneously monitored so as to detect anddetermine the location of the magnetically resonant passive tags.

Other systems have been developed for proximity detection of resonatortags for Electronic Article Surveillance (EAS) systems. The requirementsfor EAS systems are to detect the presence of a security tag within asix-foot wide aisle using one antenna assembly for both excitation anddetection of the tag within the aisle. Some EAS systems utilize tunedresonant excitation source coil drive circuitry for pulsed resonator tagoperation. As an example, U.S. Pat. No. 5,239,696 to Balch et al.discloses a linear amplifier using current feedback linear poweramplifiers to drive an excitation source tuned to resonant coils for usein pulsed EAS systems. The current feedback is used to adjust the linearamplifier's drive current level provided to the tuned excitation sourcecoil load. The current feedback is also used to provide for a relativelyconstant current drive for exciting resonant EAS tags in the field. Thesource coil is tuned to allow for use of a simple, low voltage linearamplifier circuit design. The source coil current pulse waveform isdetermined by the summation of the sinusoidal control signal and thedrive current feedback signal input to the linear amplifier.

U.S. Pat. No. 5,640,693 to Balch et al. discloses the use of linearpower amplifiers to drive excitation source coils for use in pulsed EASsystems. An apparatus for switching power to a linear amplifier isprovided to turn to an “on” state and an “off” state used to control theoutput drive pulse burst of the tuned excitation source coils. Balch etal. '693 also identifies that linear amplifiers which generate drivesignals for a source coil since linear amplifiers are typically onlyabout thirty to forty percent efficient. The inherent inefficiency ofthe linear amplifier drive is improved by switching the amplifier power“on” and “off” at the same time that the pulse control input signal tothe power supply is switched to an “on” and “off” position.

U.S. Pat. No. 5,815,076 to Herring teaches one or more damping circuitsprovided in series with excitation source coils and used to promoterapid dampening of the pulsed excitation interrogation signals at theend of each signal pulse. Providing the switchable damping circuits inseries with the antennas increases the power dissipation of the deviceduring pulse delivery due to added damping circuit switch resistance inthe antenna current path.

The above systems employ a resonator circuit energized with anexcitation signal and the resonator response signal is measured withsensing coils. The amplitude of the pulsed excitation signal requiredfor these applications is relatively low since either the resonatorcircuit to be located is of a large size, the volume in which theresonator must be located is relatively small, or the accuracyrequirements locating the resonator are quite low.

SUMMARY OF THE INVENTION

Under one aspect of the invention, a system is provided for generating amagnetic field for excitation of a leadless marker assembly. The systemof at least one embodiment includes a source generator that generates aplurality of independently controlled, alternating electrical signalseach having an adjustable phase relative to each other. A plurality ofexcitation coils are independently coupled to the source generator, andeach excitation coil has a coil axis axially misaligned with the coilaxis of the other excitation coils. The excitation coils are configuredto simultaneously receive a respective one of the alternating electricalsignals at a selected phase to generate a magnetic field. The phase ofthe alternating electrical signal for each excitation coil isindependently adjustable relative to the phase of the alternatingelectrical signal for the other excitation coils so as to adjust themagnetic field from the respective coil. The magnetic fields from theexcitation coils combine to form a spatially adjustable excitation fieldfor excitation of the remote leadless marker assembly.

Another embodiment includes a marker system with a switching networkcoupled to an energy storage device. A plurality of excitation coils areindependently coupled to the switching network. Each of the excitationscoils has a coil axis that is non-concentric with the coil axis of theother excitation coils. The excitation coils are configured tosimultaneously receive from the switching network alternating electricalsignals at a selected phase to generate a magnetic field from therespective coil. The switching network is manipulatable to independentlychange the phase for each excitation coil. The magnetic fields from theexcitation coils are configured to combine with each other to form amodifiable excitation field having selected spatial characteristics forexcitation of the leadless marker assembly. The spatially adjustableexcitation field of this embodiment effectively avoids blind spots inthe excitation of single axis marker assemblies and therefore allows themarker assemblies to be in any orientation relative to the pulsed sourcegenerator coil assembly and still be highly energized upon activation ofthe excitation source coils.

Another embodiment is directed to a marker system with a plurality ofleadless resonating marker assemblies excitable by a magnetic excitationsignal and being configured to generate marker resonant signals at oneor more selected resonant frequencies. The excitation system isprogrammable and allows for sequential excitation of unique resonantfrequency marker assemblies that may be positioned at differentorientations relative to the source coil assembly. For example, theexcitation system could excite three unique frequency resonators each ofwhich is oriented substantially along one of the three axes (X, Y and Z)by changing the directionality state of the source coil drive before theexcitation interval of the resonator of interest and providingexcitation signals that are of the appropriate directionality. Aplurality of location sensors are remote from the marker assemblies andconfigured to receive the marker signals. A source generator assemblyhas an energy storage device, first and second switching networksconnected to the energy storage device, and first and second excitationcoils. The first and second excitation coils are substantially coplanar.The first switching network and the first excitation coil areinterconnected and independent of the second excitation coil. The secondswitching network and the second excitation coil are interconnected andindependent of the first excitation coil.

Another embodiment is directed to a method of energizing a leadlessmarker assembly. The method includes directing alternating electricalsignals via a switching network through a plurality of excitation coilsthat are axially misaligned relative to each other to generate aplurality of magnetic fields having selected phases so the magneticfields combine to form a shaped excitation field. The method alsoincludes energizing a leadless resonating marker assembly with thespatially adjustable excitation field.

Another embodiment is directed to another method of energizing aleadless marker assembly. The method includes directing alternatingelectrical signals having an independently adjustable phase through aplurality of excitation coils to generate a plurality of magnetic fieldsthat combine to form a spatially adjustable excitation field. The methodalso includes energizing a leadless resonating marker assembly with thespatially adjustable excitation field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic isometric view of a system for energizing andlocating leadless miniature markers in accordance with an embodiment ofthe present invention.

FIG. 2 is an isometric view of an embodiment of an implantable miniatureresonating marker assembly usable in the system of FIG. 1.

FIG. 3 is a schematic block diagram of the excitation system of FIG. 1illustrating a source signal generator, a signal processing device, asensor array, and leadless markers.

FIG. 4 is a schematic voltage diagram showing the voltage across asource coil of the system of FIG. 1 during excitation phases over timefor the continuous wave (CW) excitation and location of the markerassembly.

FIG. 5 is a schematic bipolar current diagram during the excitationphase and over time of FIG. 4 for the continuous wave (CW) excitationand location of the marker assembly.

FIG. 6 is a schematic block diagram of an embodiment of the sourcesignal generator of FIG. 3.

FIG. 7 is a schematic voltage diagram showing the voltage across asource coil of the system of FIG. 1 during excitation “on” andexcitation “off” phases over time for the pulsed excitation and locationof the marker assembly.

FIG. 8 is a schematic bipolar current diagram during the excitation “on”and excitation “off” phases over time of FIG. 7 for pulsed excitationand location of a marker assembly.

FIG. 9 is a schematic electrical diagram of an alternate embodiment ofthe system of FIG. 3, the system including a pulse extinguishingcircuit.

FIG. 10 is a schematic isometric view of an alternate embodiment of thesystem for energizing and locating leadless miniature markers.

FIG. 11 is a schematic block diagram of the excitation system of FIG. 10illustrating a source signal generator with four coplanar source coils.

FIG. 12 is a schematic view of the coplanar source coils of FIG. 11carrying electrical signals in a first combination of phases to generatea first excitation field.

FIG. 13 is a schematic view of the coplanar source coils of FIG. 11carrying the electrical signals in a second combination of phases togenerate a second excitation field.

FIG. 14 is a schematic view of the coplanar source coils of FIG. 11carrying the electrical signals in a third combination of phases togenerate a third excitation field.

FIG. 15 is a schematic view of the coplanar source coils of FIG. 11illustrating one magnetic excitation field from the current flows alongthe coils for excitation of leadless markers in a first spatialorientation.

FIG. 16 is a schematic view of the coplanar source coils of FIG. 11illustrating another magnetic excitation field from the current flowsalong the coils for excitation of leadless markers in a second spatialorientation.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of theinvention. However, one skilled in the art will understand that theinvention may be practiced without these details. In other instances,well-known structures associated with magnetic excitation systems,resonating markers, and activators have not been shown or described indetail to avoid unnecessarily obscuring the description of theembodiments of the invention.

FIGS. 1-16 illustrate a system and components for generating anexcitation signal for activating a resonating marker assembly andlocating the marker in three-dimensional space in accordance withembodiments of the present invention. Several of the componentsdescribed below with reference to FIGS. 1-16 can also be used in systemsfor performing methods in accordance with aspects of the presentinvention. Therefore, like reference numbers refer to like componentsand features throughout the various figures.

FIG. 1 is a schematic isometric view of a system 10 for energizing andlocating one or more leadless resonating marker assemblies 14 inthree-dimensional space relative to a sensor array 16 in accordance withone embodiment of the present invention. The system 10 includes a sourcegenerator 18 that generates a selected magnetic excitation field orexcitation signal 20 that energizes the marker assemblies 14. Theenergized marker assemblies 14 generate a measurable marker signal 22that can be sufficiently measured in the presence of both the excitationsource signal and environmental noise sources. In the illustratedembodiment, the marker assemblies 14 are positioned in or on a selectedobject 24 in a known orientation relative to each other. The markersignals 22 are measured by a plurality of sensors 26 in the sensor array16. The sensors 26 are coupled to a signal processor 28 that utilizesthe measurement of the marker signals 22 from the sensors 26 tocalculate the location of each marker assembly 14 in three-dimensionalspace relative to a known frame of reference, such as the sensor array16.

In the illustrated embodiment, as discussed in greater detail below, thesource generator 18 is configured to generate the excitation signal 20so that one or more marker assemblies 14 are sufficiently energized togenerate the marker signals 22. In one embodiment the source generator18 can be switched off after the marker assemblies are energized. Oncethe source generator 18 is switched off, the excitation signal 20terminates and is not measurable. Accordingly, the sensors 26 in thesensor array 16 will receive only the marker signals 22 without anyinterference or magnetic field distortion induced by the excitationsignal 20. Termination of the excitation signal 20 occurs before ameasurement phase in which marker signals 22 are measured. Suchtermination of the excitation signal before the measurement phase whenthe energized marker assemblies 14 are generating the marker signals 22allows for a sensor array 16 of increased sensitivity that can providedata of a high signal-to-noise ratio to the signal processor 28 forextremely accurate determination of the three-dimensional location ofthe marker assemblies 14 relative to the sensor array or other frame ofreference.

The miniature marker assemblies 14 in the system 10 of the illustratedembodiment are inert, activatable assemblies that can be excited togenerate a signal at a resonant frequency measurable by the sensor array16 remote from the target 24. In one embodiment, the miniature markerassemblies 14 have, as one example, a diameter of approximately 2 mm anda length of approximately 5 mm, although other marker assemblies canhave different dimensions. An example of the marker detection system isdescribed in detail in co-pending U.S. patent application Ser. No.09/877,498, titled GUIDED RADIATION THERAPY SYSTEM, filed Jun. 8, 2001,which is incorporated herein in its entirety by reference thereto.

The illustrated marker assembly 14 includes a coil 30 wound around aferromagnetic core 32 to form an inductor (L). The inductor (L) isconnected to a capacitor 34, so as to form a signal element 36.Accordingly, the signal element 36 is an inductor (L) capacitor (C)resonant circuit. The signal element 36 in one embodiment is enclosedand sealed in an encapsulation member 38 made of plastic, glass, orother inert material. The illustrated marker assembly 14 is a fullycontained and inert unit that can be used, as an example, in medicalprocedures in which the marker assembly is secured on and/or implantedin a patient's body.

Other marker assemblies 14 are described in detail in co-pending U.S.patent application Ser. No. 09/954,700, filed Sept. 14, 2001, which isalso incorporated herein in its entirety by reference thereto. Otherleadless marker assemblies 14 that may be used are described in detailin U.S. Pat. No. 6,363,940, titled SYSTEM AND METHOD FOR BRACKETING ANDREMOVING TISSUE, filed May, 14, 1998; PCT Application No. PCT/US00/31667titled SYSTEMS AND METHODS FOR LOCATING AND DEFINING A TARGET LOCATIONWITHIN A HUMAN BODY, filed Nov. 17, 2000; and PCT/US00/31673, titledSYSTEMS AND METHODS FOR STABILIZING A TARGET LOCATION WITHIN A HUMANBODY, filed Nov. 17, 2000, all of which are incorporated herein in theirentireties by reference thereto.

The marker assembly 14 is energized, and thus activated, by the magneticexcitation field or excitation signal 20 generated by the sourcegenerator 18 such that the marker's signal element 36 generates themeasurable marker signal 22. The strength of the measurable markersignal 22 is high relative to environmental background noise at themarker resonant frequency, thereby allowing the marker assembly 14 to beprecisely located in three-dimensional space relative to the sensorarray 16.

The source generator 18, in one embodiment, is adjustable to generate amagnetic field 20 having a waveform that contains energy at selectedfrequencies that substantially match the resonant frequency of thespecifically tuned marker assembly 14. When the marker assembly 14 isexcited by the magnetic field 20, the signal element 36 generates theresponse marker signal 22 containing frequency components centered atthe marker's resonant frequency. After the marker assembly 14 isenergized for a selected time period, the source generator 18 isswitched to the “off” position so the pulsed excitation signal 20 isterminated and provides no measurable interference with the markersignal 22 as received by the sensor array 16.

The marker assembly 14 is constructed to provide an appropriately strongand distinct signal by optimizing marker characteristics and byaccurately tuning the marker assembly to a predetermined frequency.Accordingly, multiple uniquely tuned, energized marker assemblies 14 maybe reliably and uniquely measured by the sensor array 16. The uniquemarker assemblies 14 at unique resonant frequencies may be excited andmeasured simultaneously or during unique time periods. The signal fromthe tuned miniature marker assembly 14 is significantly aboveenvironmental signal noise and sufficiently strong to allow the signalprocessor 28 (FIG. 1) to determine the marker assembly's identity,precise location, and orientation in three dimensional space relative tothe sensor array 16 or other selected reference frame.

The system 10 of the illustrated embodiment in FIG. 1 can be used inmany different applications in which the miniature marker's precisethree-dimensional location within an accuracy of approximately 1 mm canbe uniquely identified within a relatively large navigational orexcitation volume, such as a volume of 12 cm×12 cm×12 cm or greater. Onesuch application is the use of the system 10 to accurately track theposition of targets (e.g., tissue) within the human body. In thisapplication, the leadless marker assemblies 14 are implanted at or nearthe target so the marker assemblies move with the target as a unit andprovide positional references of the target relative to a referenceframe outside of the body. Such a system could also track relativepositions of therapeutic devices (i.e., surgical tools, tissue ablationdevices, radiation delivery devices, or other medical devices) relativeto the same fixed reference frame by positioning additional leadlessmarker assemblies 14 on these devices at known locations or bypositioning these devices relative to the reference frame. The size ofthe leadless markers used on therapeutic devices may be increased toallow for greater marker signal levels and a corresponding increase innavigational volume for these devices.

The system 10 is configured to generate a sufficient source excitationmagnetic field signal, as an example, source excitation coil MagneticDipole Moment approximately greater than 0.5 A*m², at the appropriatefrequency to energize the one or more marker assemblies significantlyabove the environmental background noise. In one embodiment, the system10 also generates excitation source magnetic field flux signals atselected spatial orientations (e.g., so the magnetic moment issubstantially oriented along the X, Y and Z axes). The magnetic fieldflux signals excite a marker assembly 14 that may be a resonator havinga single coil (i.e., a single-axis coil) substantially oriented along asingle axis and spatially orthogonal to excitation energy along theother two axes. The system 10 can also be configured to generateexcitation source magnetic field flux signals at variable frequencies toenergize a plurality or marker assemblies 14 that contain resonantcircuits tuned to these unique frequencies. Accordingly, the multiplemarker assemblies 14 can be sequentially or simultaneously energized.

The system 10 is also configured to avoid the difficult process ofseparating the small marker signal 22 from the much more powerful sourceexcitation signal 20 by measuring the marker signal during a time periodwhen the source signal is substantially not present. The residual sourceexcitation signal 20 can cause substantially less accurate results inthe location determination if not controlled. The system 10 provides forthe measurement of the marker signal 22 with a sufficient number ofspatially distributed sensors 26 at a sufficient signal-to-noise ratioso that an inversion algorithm provided in the signal processor 28 canaccurately solve for the three-dimensional location of the particularmarker assembly 14 relative to the known reference frame of the sensorarray 16.

FIG. 3 is a schematic block diagram of the system 10 in accordance withone embodiment. As seen in FIG. 3, the plurality of leadless miniaturemarker assemblies 14 on the target 24 are remote from the sourcegenerator 18 and from the sensor array 16. The sensor array 16 isoperatively coupled to the signal processing device 28.

The source generator 18 includes a high voltage power supply 40 coupledto an energy storage device 42. In one embodiment, the power supply 40is a 500 volt power supply, although other power supplies with higher orlower voltages can be used. The energy storage device 42 in oneembodiment is a high voltage capacitor that can be charged andmaintained at a relatively constant charge by the power supply 40.Energy stored in the storage device 42 allows for providing energy toand removing energy from the source coil inductor. A small amount ofstored energy is present in the source coil inductor at maximum currentrelative to the energy stored in the energy storage device 42.

The energy storage device 42 is capable of storing adequate energy tominimize voltage droop in the energy storage device while having a lowseries resistance so as to minimize power losses. The energy storagedevice 42 also has a low series inductance to allow for maximum sourcegenerator excitation capability to drive the source coil 46. Specializedaluminum electrolytic capacitors used in flash energy applications maybe used in one of the embodiments of system 10. Alternative energystorage devices can also include NiCd and lead acid batteries, as wellas alternative capacitor types, such as tantalum, film, or the like.

The source generator 18 of the illustrated embodiment also includes aswitching network 44 coupled between the energy storage device 42 and aplurality of untuned excitation source coils 46. The switching network44 is configured to control the polarity of the voltage across thesource coils 46 and the resultant current magnitude and polarity withtime through the source coils so that the source coils 46 each generatethe high energy excitation field 20 for activating the marker assemblies(FIG. 1).

In the illustrated embodiment, the excitation source coils 46 includesthree coils orthogonally oriented to correspond to the X, Y, and Z axesof a selected frame of reference. The three coils will be referred toherein as the X coil 48, Y coil 50, and Z coil 52, each of which isconfigured to provide a magnetic field flux signal along the respectiveX, Y, and Z axes. The X, Y, and Z coils 48, 50, and 52 with theirorthogonal orientation effectively avoid blind spots for excitation ofmarker assemblies 14 and can allow the marker assemblies to be in anyorientation relative to the source generator 18 and still be highlyenergized upon activation of the excitation source coils 46.

The source coil 46 in the illustrated embodiment is configured todeliver adequate source excitation magnetic fields, which is defined bythe area, number of turns, current, and other characteristics of thecoil. The source coil 46 is constructed with conductors designed tominimize power loss due to resistive losses as well as resistive lossesdue to skin effects. Examples include small diameter wire wound in aLITZ wire configuration to minimize skin effects, or alternatively, athin sheet of conductor to minimize skin effects. Parasitic interwindingcapacitance of the source coil inductor and interconnection conductorsshould also be minimized to reduce unintended coil current spikes due tovery short switching network voltage transition times and otherunintended source coil resonant circuit affects.

The switching network 44 in the illustrated embodiment includes aplurality of switches that control energy flow to the X, Y, and Z coils48, 50 and 52. These switches include X-switches 54 coupled to theX-coil 48, Y-switches 56 coupled to the Y-coil 50, and Z-switches 58coupled to the Z-coil 52. The X-switches 54 include a plurality ofactivatable switch mechanisms that control the polarity of the voltageacross the X coil 48 so as to control the polarity of the derivativewith time of the current flow through the X coil. The Y switches 56control the polarity of the voltage across the Y coil 50 so as tocontrol the polarity of the derivative with time of the current flowthrough the Y coil. Similarly, the Z-switches 58 are configured tocontrol the polarity of the voltage across the Z coil 52 so as tocontrol the derivative with time of the current flow through the Z coil.The derivative with time of the current flow at a particular voltagepolarity across a particular source coil results in either an increasingor decreasing current ramp with time through the source coil.

In the illustrated embodiment, each of the X, Y, and Z switches 54, 56,and 58 have an H-bridge configuration with four switch mechanismscontrolled to selectively direct electrical current through therespective X, Y, or Z coil 48, 50, and 52, thereby generating a pulsedmagnetic field from the respective source coil.

The X, Y, and Z switches 54, 56, and 58 are also each configured toalternately switch between first and second “on” positions to generate acurrent flow with a bipolar waveform. In the first “on” position, thecurrent flow in one has a continually increasing current ramp with timethrough the respective X, Y, or Z coil 48, 50 or 52 to generate themagnetic excitation field. In the second “on” position, the current flowhas a continually decreasing current ramp with time through therespective X, Y, or Z coil 48, 50 or 52 to generate the magneticexcitation field. Such alternate switching over the pulse waveformperiod effectively provides for alternating the polarity of the currentflow from a positive polarity to a negative polarity. The X, Y, and Zswitches 54, 56, and 58 are also configured to alternately transferstored energy from the energy storage device 42 to the respective X, Y,or Z axis source coil 48, 50, or 52 and to transfer the stored energyfrom the respective source coil back to the energy storage device whenalternately switching between the first and second “on” positions.

The X, Y, and Z switches 54, 56, and 58 in one embodiment are alsoconfigured to move to an “off” position to prevent energy transfer fromthe energy storage device 42 to the respective X, Y, or Z coils 48, 50,or 52. Once the X, Y, or Z switch 54, 56, or 58 is switched to the “off”position at the appropriate time when the energy (i.e. the current) inthe source coil is substantially zero, the respective X, Y, or Z coil48, 50, or 52 no longer generates a magnetic field such that the pulsedexcitation signal 20 ceases, so only the marker signal 22 emanating fromthe marker assembly 14 is measurable.

In an alternate embodiment, the source generator 18 is configured toprovide a continuous wave excitation signal as the X, Y, and Z switches54, 56, and 58 alternatively switch between the first and second “on”positions to energize the leadless marker assemblies 14. FIG. 4illustrates a schematic voltage diagram showing a bipolar voltage acrossa source coil 64 to generate the continuous excitation signal, and FIG.5 is a schematic bipolar current diagram of the continuous excitationsignal corresponding to the voltage diagram of FIG. 4.

The X, Y, and Z coils 48, 50, and 52 are untuned coils so that thefrequency of the excitation signal 20 can be changed or modified tomatch the different resonant frequencies of a plurality of markerassemblies 14. The frequency of the excitation signal 20 can be changedafter the marker assembly 14 is energized. The sensor array 16 and thesignal processor 28 can distinguish the marker signal 22 from theexcitation signal 20 by measuring the marker resonant signal while thesource signal is substantially present but of a different frequency fromthe measured marker frequency. Accordingly, the source signal generatorexcitation waveform is substantially present during measurement of theleadless marker signal 22.

The use of a highly energy efficient source generator 18 that drives anuntuned source coil allows for dynamic adjustment of source coil'scontinuous waveform excitation signal frequency without adjusting ormodifying the tuning capacitor of a tuned resonant source coil circuit.This feature allows for adjustment of the source coil's continuouswaveform excitation signal frequency to excite multiple uniquely tunedmarker resonators during different time periods from the same sourcecoil without modifying a source coil tuning capacitance that is presentin a tuned source coil embodiment. Furthermore, an untuned source coilembodiment is not susceptible to source detuning effects of tuned sourcecoil embodiments.

In another embodiment, the source frequency may be adjusted such thatthe marker signal 22 is a lagging 90 degrees phase difference with theexcitation signal 20 when the marker assembly 14 is excited at theresonant frequency of the marker assembly. Accordingly, the signal canbe distinguished from the marker signal because of the phase differencebetween the source and one or more marker signals.

Therefore, different embodiments of the invention may distinguish themarker signal from the source signal because of time, phase or frequencydifferences between the source and one or more marker signals.

The alternating current flow through the X, Y, or Z coil 48, 50, or 52generates the pulsed magnetic field in the selected axis to excite themarker assembly 14 located at a determinable distance from the coil. Thefollowing is an equation for calculation of the on-axis magnetic fieldexcitation at a distance from a source coil Inductor of a solenoidshape:${B_{marker}\quad({distance})} = \frac{µ_{o} \cdot I_{source} \cdot r_{source}^{2} \cdot N_{source}}{2 \cdot \left( {r_{source}^{2} + {Distance}^{2}} \right)^{\frac{3}{2}}}$Where:

-   -   B_(marker)(distance)=magnetic field flux density at a distance        from the source coil along the coil axis    -   μ_(o)=permeability of free space between the source coil and the        marker assembly    -   r_(source)=Radius of the source coil (meters)    -   N_(source)=Number of turns of wire around the source coil    -   Distance=distance from the source coil (meters)    -   I_(source)=Electrical current through source coil (amperes)

The electrical current (I_(source)) through the X, Y, or Z coil 48, 50,or 52 is determined by the voltage applied across the coil inductance(L). The inductance (L) of an air core coil (solenoid) is approximatedby:$L = \frac{\left( {µ_{o} \cdot \pi \cdot r_{source}^{2} \cdot N_{source}^{2}} \right)}{{length}_{source}}$Where:

-   -   L=Inductance of the source coil    -   μ_(o)=permeability of free space    -   r_(source)=Radius of the source coil (meters)    -   N_(source)=Number of turns of wire around the source coil    -   length_(source)=Length of source coil (meters)

The inductance of the source coil determines the electrical currentderivative with time through source coil inductor as follows:V=L*dl/dt or V/L=dl/dtWhere:

-   -   V=voltage potential across the coil (volts)    -   L=Inductance of the source coil    -   dl/dt=The change in coil current with time

To efficiently transfer energy to a coil and create the magneticexcitation field, power losses due to resistance in the source coilcircuit should be minimized. The power (i.e., RMS power) in the systemis determined as follows:P=I²R_(TOTAL)Where:

-   -   I=Root Mean Square value of the current    -   R_(TOTAL)=Total resistance in the source coil circuit

The power losses of the system 10 limits the strength of the magneticexcitation field that the system is capable of delivering. When themarker excitation distance is large compared to the effective radius ofthe magnetic field source, the strength of the magnetic excitation fielddecreases with the cube the distance from the magnetic field source(e.g., the source coil 46). When the marker excitation distance is notlarge compared to the effective radius of the magnetic field source, thestrength of the magnetic excitation field decreases much less rapidlywith the distance from the magnetic field source (e.g., the source coil46). Furthermore, the magnetic excitation field increases linearly withcurrent, but the power dissipation in the system increases as the squareof the current. Accordingly, the requirements for source driveelectronics become more challenging to allow for adequate levels of themarker signal 22 as the magnetic excitation field requirements increase.

In one embodiment, the source coil 46 can be operated as a tuned circuitwherein the source coil inductor is tuned with a capacitor chosen forthe desired resonant frequency of the marker assembly 14. The reactanceof the inductor is equal to and opposite the capacitor at the frequencyof interest, such that only the resistance of the source coil circuit isseen by the drive electronics. Substantial stored energy can occur inthe tuned circuit, however, that may limit the time in which the sourcecoil excitation signal is shutoff. Longer shut-off time reduces the timeduring which the marker resonator signal can be measured effectively inthe absence of the source signal. Accordingly, a shorter shutoff timecan be very desirable. A shorter shutoff time also allows for adequatemarker ring down signal to remain after the source signal has decayedsuch that the marker signal may be measured at a sufficient signal tonoise ratio for accurate location of the marker.

The system 10 of the one embodiment provides an untuned source coil 46configured to excite the leadless marker assembly's resonator with thepulsed excitation signal 20 having a high level of source excitationenergy. The system 10 also provides a very short source excitationturnoff time to achieve optimal performance for measuring the markersignal 22 after the source excitation signal 20 is significantlyextinguished. Accordingly, the turnoff time for an untuned source coilcan have a significant impact on the effectiveness of the system 10, sothe system is configured to have only limited stored energy afterturnoff time. Therefore, the source generator 18 of the system 10 isconfigured to deliver high source coil current amplitudes at the optimalmarker excitation frequencies to energize the leadless markers 14 beforethe time decaying marker ring down signal is measured.

FIG. 6 is a schematic block diagram of a source generator 18 of oneembodiment. In this illustrated embodiment, a high-voltage power supply40 and energy storage device 42 are shown coupled to a single switchnetwork 44, which has a single H-bridge switching configuration. Thisswitch network 44 has four switches 62 a-d coupled together so as tocontrol the current flow to a single source coil 64. In this embodiment,when multiple source coils are used, each source coil is coupled to aseparate switch network 44, power supply 40, and energy storage device42. In one embodiment, a common energy storage device 42 and a commonpower supply 40 for multiple switching networks 44 and correspondingsource coils are used, as shown in FIG. 3.

The switch network 44 of the illustrated embodiment with the H-bridgeconfiguration is constructed of MOSFET devices. The switching deviceshave a low “on” series of resistance (less than or equal toapproximately 1 ohm) to minimize power losses. Furthermore, very shortswitching times (less than approximately 25 nanoseconds) are provided tominimize switching induced power losses and maximum excitation delivery.The short switching times allow for high frequency system operation (forexample, 50-600 kHz) that improves overall system performance due toincreases in inductive coupling efficiencies between the source, marker,and sensor coils with frequency. Alternate embodiments have switchnetworks 44 that include one-half H-bridge configurations (having onlytwo switches) with a matched set of power supplies and two matchedenergy storage devices configured to deliver a bipolar voltage drive tothe source coil 46. The alternative switching components can includebipolar junction transistors (BJT), JFETs, and various possible vacuumtube configurations.

The power supply 40 directs charge to and maintains a relativelyconstant energy storage level at the energy storage device 42 bydelivering average power to the energy storage device. The amplitude ofthe charging current is large enough to compensate for average powerconsumption in the pulse generator and keep the energy storage device ata relatively constant voltage. The energy storage capacitor must belarge enough to minimize voltage droop during pulse generation andcapable of both supplying high instantaneous power to the switch networkand returning high instantaneous power from the switch network 44. Thefollowing example for one embodiment has equations for both calculationof the power supply average power and pulse generator instantaneouspower delivery: V_(cap) = 500 V Capacitor charge voltage R_(switching) =2.0 ohm Total resistance in “on” state (2 switches closed at a time)R_(coil) = 0.25 ohm Source coil resistance R_(energy) _(—) _(storage)_(—) _(device) = 0.05 ohm Energy storage device resistance I_(L)(rms) =4.8 amps Source coil rms current (during pulsing) Duty cycle = 50% % oftime generator is pulsing Average power dissipation = (R_(switching) +R_(coil) + R_(energy) _(—) _(storage) _(—) _(device)) * I_(L)(rms)² *Duty cycle P_(Power) _(—) _(supply) (average) = (2 + 0.25 + 0.05) *(4.8)² * 0.5 Watts P_(Power) _(—) _(supply) (average) = 26 wattsP_(instantaneous) (during pulsing) = 500 V * 4.8 amps RMS = 2,400 VA RMSTo maintain constant energy storage:I _(Power) _(—) _(supply)(average)=P _(Power) _(—) _(supply)(average)/V_(cap)I _(Power) _(—) _(supply)(average)=26 W/500V=52 milli Ampere

The switch network 44 is configured so pairs of the switches 62 a-d areopened and closed simultaneously when the other pair of switches aresimultaneously closed and opened. As a result, the switch network 44 isconfigured to control the voltage across the source coil 64 inalternating polarities and at selected phases relative to each other. Asa result, the current flow through the source coil 64 alternates betweenpositive and negative polarity over the pulsed magnetic excitationsignal waveform pulse burst time period. This alternating of the currentflow through the source coil between positive and negative polarityresults in the pulsed magnetic excitation signal 20 from the sourcecoil.

In one embodiment, the switch network 44 moves between the first “on”position when switches 62 b and 62 c are closed and switches 62 a and 62d are open, and the second “on” position when switches 62 b and 62 c areopen and switches 62 a and 62 d are closed. In this first “on” position,the voltage having a positive direction across the source coil 64increases the current over the pulse “on” time through the source coilin the direction of arrow 65. In the second “on” position, the voltagehaving a negative direction across the source coil 64 decreases thecurrent over the pulse “on” time through the source coil in the oppositedirection of arrow 65. The resulting pulsed excitation signal due tomultiple pulses over the pulse burst time period and the repetitionfrequency of the pulse burst has determinable frequency componentscorresponding to the resonant frequencies of the marker assemblies 14for activation of the markers (not shown).

The switch network 44 in the one embodiment also includes an “off”position that opens the circuit to prevent the current from flowing tothe source coil. Accordingly, no magnetic field is generated and nopulsed excitation signal 20 is emanated from the source coil 64. Theswitch network 44 can be switched to the “off” position by openingswitched 62 a and 62 b and by turning 62 c and 62 d to a closed positionto shunt the source coil which allows for exponential decay of residualenergy in the source coil. The current then circulates through one ofthe two switches 62 c and 62 d and one of two H-bridge protection diodes67 coupled to the switches depending on the polarity of the residualcurrent. The residual current in the source coil decays by the followingexponential time constant:τ=L _(sc)/(R _(L) +R _(switch) +R _(protection) _(—) _(diode))where:

-   -   L_(sc)=Source coil inductance    -   R_(L)=Source coil resistance    -   R_(switch)=Switch “on” resistance    -   R_(protection) _(—) _(diode)=Protection diode “on” resistance

As a result, the switch network 44 is controllable to control theexcitation signal 20 and can be switched to the “off” position toterminate the excitation signal after the marker assemblies have beensufficiently energized. When the excitation signal is off, the markersignals 22 do not have to be distinguished from the excitation signal 20when the marker signal is being measured and analyzed as discussedabove.

In an alternate embodiment, the switch network 44 can be switched to the“off” position by opening by opening all switches 62 a-d. However, caremust be taken to assure that little residual energy remains in thesource coil 64 at the time all switches are turned “off”, otherwiseinductive kick back induced voltage spikes due to an instantaneouscurrent change through the inductor with time may damage the switchnetwork or result in undesired oscillations of the residual current inthe source coil.

FIG. 7 is a schematic voltage diagram showing the bipolar voltage acrossa source coil 64 to generate the pulsed excitation signal. The voltageis controlled by activating the switch network 44, as discussed abovewith respect to FIG. 6. When the switch network 44 is switched to thefirst “on” position, the voltage across the source coil 64 is positiveas shown in FIG. 7 at reference number 66, thereby providing a positivepolarity. When the switch network 44 is switched to the second “on”position, the voltage across the source coil 64 shifts and is negativeas shown in FIG. 7 at reference number 68, thereby providing a negativepolarity.

The switch network 44 (FIG. 6) alternates between the first and second“on” positions so as to alternate the voltage across the source coil 64between the positive and negative polarities. When the switch network 44is switched to the first “on” position, energy is drawn from thecapacitor 42 toward the source coil 64. When the switch network 44 isswitched to the second “on” position, energy is directed back to thecapacitor 42 until the time when the current changes polarity and thenenergy is again drawn from the capacitor. When the switch network 44 isswitched back to the first “on” position, then energy is again directedback to the capacitor 42 until the time when the current changespolarity again and then energy is again drawn from the capacitor. As aresult, the switching network 44 is configured to alternately transferstored energy from the energy storage device 42 to the source coil 64and to transfer the storage energy from the source coil back to theenergy storage device when alternately switching between the first andsecond “on” positions.

In one embodiment, the switch network 44 is configured so that theswitches 62 a-d (FIG. 6) are in the first “on” position for the sametotal time period for a particular pulse burst that they are in thesecond “on” position. Accordingly, the time period at the positivepolarity is the same time period as the negative polarity for thevoltage across the source coil 64. In the embodiment, when the switchnetwork 44 is switched to the “off” position, the voltage across thesource coil 64 is zero, as reflected at reference 70 in the voltagediagram. When the current through the source coil 64 (FIG. 6) is zero,the source coil provides no excitation signal 20 that would interferewith the marker signal 22 generated by the marker assembly 14 asdiscussed above.

FIG. 8 is a schematic bipolar current diagram corresponding to thevoltage diagram of FIG. 7. Referring to FIGS. 6, 7 and 8, when theswitch network 44 (FIG. 6) is in the first “on” position correspondingso the voltage has a positive polarity 66 (FIG. 7), the current flowthrough the source coil 64 (FIG. 6) has a continually increasing currentramp 70 (FIG. 8) so as to generate the magnetic excitation field 20 withthe positive polarity. When the switch network 44 (FIG. 6) is in thesecond “on” position corresponding to the negative polarity 68 (FIG. 7),the current flow has a continually decreasing current ramp 72 (FIG. 8).When the switch network 44 switches back to the first “on” position, thecurrent flow switches back to an increasing current ramp 70.Accordingly, the current flow has a bipolar, generally non-sinusoidaltriangular shaped waveform.

As seen in FIG. 8, when the current flow switches directions, forexample from the decreasing current ramp 72 to the increasing currentramp 70, there is a point shown at 74 at which the current in the sourcecoil is substantially at zero amps. Similarly, when the current flow hasthe decreasing current ramp 72, there is a point shown at 76 at whichthe current in the source coil 64 (FIG. 6) is at zero amps. The switchnetwork 44 is configured to switch to the “off” position when thecurrent flow through the source coil 64 is substantially at these zeropoints 74 and 76, and thus at zero amps. When the current in the sourcecoil 64 approaches zero amps, the signal decay time for the pulsedexcitation signal 20 approaches zero, such that the pulsed excitationsignal is substantially instantaneously shut off so as to provide nointerference with the marker signal 22 generated by the energized markerassembly 14.

In the embodiment wherein the excitation signal 20 is a CW signal, theswitch network 44 continually switches between the first and second “on”positions during the time period when the marker signal is measured. Inthe pulsed excitation signal embodiment wherein the excitation signal isterminated when the switch network 44 is switched to the “off” position,the extent of signal decay over time of the source coil 64 current is afunction of the resistance in the switch network as well as resistanceand inductance in the source coil circuitry. In the illustratedembodiment, the switch network 44 and source coil 64 are configured suchthat when the switch network is switched to the “off” position when thestored energy in the source coil is minimized.

FIG. 9 is a block diagram of a source signal generator 90 having a pulseextinguisher circuit 100 therein in accordance with an alternateembodiment. In this alternate embodiment, the source signal generator 90includes the power supply 40 and the energy storage device 42 coupled tothe switch network 44 similar to the embodiments discussed above. Theswitch network 44 is also coupled to the source coil 64. The switchnetwork 44 is also connected to the pulse extinguisher circuit 100 inparallel with the source coil inductor, such that the pulse extinguishercircuit does not conduct current during pulse generation. The sourcesignal generator 90 of this alternate embodiment provides the switchnetwork 44 moveable between the first “on” position, the second “on”position as discussed above, and an “off” position where all of theswitches, 62 a-62 d, are opened. When the switch network 44 is switchedto the “off” position, the pulse extinguisher circuit 100 switches 104are closed to quickly dissipate the residual energy in the source coil64 so as to limit the time that an excitation field continues to begenerated after the switch network has been moved to the “off” position.

The pulse extinguisher circuit 100 is configured to include a pair ofresistors 102 and switches 104 that can be activated to quicklydissipate the residual energy in the source coil when the circuit isclosed and to interrupt the current flow through the extinguishercircuit when the switches 104 are open during excitation pulsegeneration. At the end of a pulse burst, the residual source coilcurrent is conducted through one of the two switches 104, one of the tworesistors 102 and an H-bridge protection diode 105, depending on thepolarity of the residual current. Pairs of switches 104 and resistors102 are provided so the residual current may be either of a positive ornegative polarity. The time constant for de-energizing the source coilinductor is determined by the following equation:τ=L _(sc)/(R _(L) +R _(switch) +R _(protection) _(—) _(diode) +R_(pulse) _(—) _(extinguisher))Where:

-   -   L_(sc)=Source coil inductance    -   R_(L)=Source coil resistance    -   R_(switch)=Switch “on” resistance    -   R_(protection) _(—) _(diode)=Protection diode “on” resistance    -   R_(pulse) _(—) _(extinguisher)=Pulse extinguisher resistor        resistance

The pulse extinguisher circuit's resistors 102 decrease the exponentialdecay time constant of residual current in the source coil by addingadditional resistance to the source coil circuit only during turnofftime. As a result, the pulse extinguisher circuit 100 decreases theturnoff time of the source generator and does not decrease the overallenergy efficiency or power dissipation of the source signal generator90.

In the illustrated embodiment, the pulse extinguisher circuit 100 isconfigured so that no measurable or detectable excitation field remainsafter the switch network 44 has been moved to the “off” position withinless than one cycle of the resonating marker assembly 14 (FIG. 1).Accordingly, the pulse extinguisher circuit 100 facilitates thetermination of the pulsed excitation signal 20 so that the resonatingmarker assembly 14 can be easily detectable and located with the signalprocessing assembly 28 to accurately identify the three-dimensionallocation of the resonating marker assembly relative to the sensor array16.

While the pulse extinguisher circuit 100 is defined in the illustratedembodiment as providing the resistors 102 and switches 104, withadditional protection diodes 105, other pulse extinguisher circuits canbe used so as to increase the resistance of the total source coilcircuit when the switch network is in the “off” position. The alternatepulse extinguisher circuits should not increase system power dissipationand decrease energy efficiency by adding additional switch resistanceduring pulse delivery because of its presence.

FIG. 10 is an isometric view of an alternate embodiment of the system 10for energizing and locating miniature leadless markers 14. In thisalternate embodiment, the pulsed source generator 18 includes aplurality of excitation source coils 46 adjacent to each other and eachindependently coupled to the source generator. Each source coil 46 hasprimary coil axis 200 axially misaligned with the coil axis of the othersource coils. In the illustrated embodiment, the plurality of sourcecoils 46 include coils 1-4, and the four coils are all coplanar. Each ofcoils 1-4 has a square shape, and coils 1-4 are arranged so as to form aflat, substantially rectangular or square coil configuration. Inalternate embodiments, other coil shapes and coil configuration shapescan be used with greater or fewer source coils 46. In one embodiment,coils 1-4 are embedded in a substantially planar substrate 202, such asa printed circuit board. In another embodiment, the coils 1-4 can beformed in separate substrates arranged so at least two of the sourcecoils are substantially coplanar.

In one embodiment, the coil axes 200 of at least two source coils 46(e.g., coils 1-4) are parallel, and in another embodiment, the coil axesof at least two source coils are nonconcentricly aligned with eachother. In the illustrated embodiment, each of source coils 46 (e.g.,coils 1-4) is adjacent to the other source coils while beingelectrically isolated from each other. Each of the source coils 46 isindependently coupled to a source generator 204, such that a pluralityof independently controlled, alternating electrical signals havingadjustable phases can simultaneously be provided to the source coils.Each of coils 1-4 are configured to simultaneously receive one of thealternating electrical signals at a selected phase so as to generate amagnetic field around the respective source coil. As discussed ingreater detail below, the magnetic fields from each of coils 1-4 combineto form a spatially adjustable excitation field that generates thesource excitation signal at the selected frequency to energize theleadless markers 14.

FIG. 11 is a schematic block diagram of the system 10 in accordance withthe alternate embodiment of FIG. 10. As seen in FIG. 11, the pluralityof leadless miniature marker assemblies 14 on the target 24 are remotefrom the source generator 18 and from the sensors 16. In this alternateembodiment, the source generator 18 includes a switch network 206 withan H-bridge switch assembly 208 for each of coils 1-4. The switchnetwork 206 is connected to the energy storage device 42, which isconnected to the high voltage power supply 204. Each H-bridge 208 isconfigured to independently control the phase of the alternatingelectrical signal received by its respective source coil 46, therebyindependently controlling the phase of the magnetic field generated byeach of coils 1-4. For example, the H-bridges 208 can be configured sothat the electrical signal for all of coils 1-4 are in phase or so theelectrical signals for one or more of coils 1-4 are 180° out of phase.Furthermore, the H-bridges 208 can be configured so that the electricalsignal for one or more of coils 1-4 are between 0 and 180° out of phase,thereby providing magnetic fields with different phases at the samepoint in time.

The selected magnetic fields from coils 1-4 combine together to form anadjustable, shaped excitation field that can have differentthree-dimensional shapes to excite the leadless markers 14 at anyspatial orientation within an excitation volume 209 positioned adjacentto coils 1-4, as shown in FIGS. 12-14. In the illustrated embodiments,wherein coils 1-4 are shown generally horizontally oriented, theexcitation volume 209 is positioned above an area approximatelycorresponding to the center of the array of source coils 46. Theexcitation volume 209 is the three-dimensional space adjacent to thesource coils 46 in which the strength of the magnetic fields at theselected frequency is sufficient to fully energize the leadless markers14 within that volume.

FIG. 12 is a schematic view of the coplanar source coils 46 (e.g., coils1-4) of FIG. 11 with the alternating electrical signals provided to thesource coils being in a first combination of phases to generate a firstexcitation field within the excitation volume 209 that providessignificant magnetic field excitation about an axis in the Y-directionrelative to the illustrated XYZ coordinate axis. Each of the coils 1-4has four straight sides, two outer sides 212 and two inner sides 214.Each inner side 214 faces toward and is immediately adjacent to an innerside of another one of the source coils, and the two outer sides 212face away from the other source coils.

In the embodiment of FIG. 12, coils 1-4 all receive an alternatingelectrical signal in the same phase at the same point in time. As aresult, the electrical current is flowing in the same direction alongcoils 1-4. Accordingly, the direction of current flow along the innersides 214 of one source coil (e.g., coil 1) is opposite to the currentflow in the inner sides of the two adjacent source coils (e.g., coils 2and 3). As a result, the magnetic fields generated along those adjacentinner sides 214 substantially cancel each other out, so the combinedmagnetic fields from coils 1-4 in this embodiment is effectivelygenerated from the current flow around the outer sides 212 of 1-4. Theresulting excitation field formed by the combination of the magneticfields from coils 1-4 has a magnetic moment 215 substantially along theZ-axis within the excitation volume. This excitation field will besufficient to energize the leadless markers 14 (FIG. 11) that areaxially aligned along the Z-axis or that are positioned so the marker'slongitudinal axis has an angular component along the Z-axis.

FIG. 13 is a schematic view of the coplanar source coils 46 (e.g., coils1-4) of FIG. 11 with the alternating electrical signals provided to thesource coils in a second combination of phases to generate a secondexcitation field with a different spatial orientation. In thisembodiment, coils 1 and 3 are in phase, and coils 2 and 4 are in phase,but coils 1 and 3 are 180 degrees out of phase with coils 2 and 4. Thedirection of current flow around each coil 46 is shown by the directionarrows 213. As a result, the magnetic fields from coils 1-4 combine togenerate an excitation field 217 generally in the Y direction within theexcitation volume 209 that provides significant magnetic fieldexcitation. Accordingly, this excitation field is sufficient to energizethe leadless markers 14 that are axially aligned along the Y-axis orthat are positioned so the marker's longitudinal axis has an angularcomponent along the Y-axis.

FIG. 14 is a schematic view of the coplanar source coils 46 (e.g., coils1-4) of FIG. 11 with the alternating electrical signals being providedto coils 1-4 in a third combination of phases to generate a thirdexcitation field with a different spatial orientation. In thisembodiment, coils 1 and 2 are in phase, and coils 3 and 4 are in phase,but coils 1 and 2 are 180 degrees out of phase with coils 3 and 4.Accordingly, the magnetic fields from coils 1-4 combine to generate anexcitation field 219 in the excitation volume 209 generally along theX-axis within the excitation volume. Accordingly, this excitation fieldis sufficient to energize the leadless markers 14 that are axiallyaligned along X-axis or that are positioned so the marker's longitudinalaxis has an angular component along the X-axis.

FIG. 15 is a schematic view of the coplanar source coils 46 of FIG. 10illustrating the magnetic excitation field and the current flow togenerate the excitation field 224 for excitation of the leadless markers14 in a first spatial orientation. In this illustrated embodiment, twoleadless markers 14 are oriented within the excitation volume 209 spacedabove coils 1-4 such that the longitudinal axis of each of the leadlessmarkers is substantially parallel to the planar coils and aligned withthe Y-axis within the excitation volume. The switch network 206 (FIG.11) is configured so the phase of the alternating electrical signalsprovided to coils 1-4 are similar to the configuration of FIG. 13 so asto generate an excitation field represented by field lines 224 withinthe excitation volume 209.

The excitation field 20 (FIG. 10) is spatially adjustable to create adifferent excitation field by adjusting the switch network 206 (FIG. 10)to change the phase of the electrical signals in one or more of coils1-4, so as to energize any leadless markers 14 at different spatialorientations. As an example, FIG. 16 is a schematic view of the coplanarsource coils alternating electrical signals with phases different fromthe embodiment of FIG. 15 and the resulting excitation field 220 forexcitation of the leadless markers 14 within the excitation volume 209.In this embodiment, the current flow through coils 1-4 is similar tothat as shown in FIG. 12, such that the excitation field 220 within theexcitation volume 209 is at a spatial orientation so as to excite theillustrated leadless markers 14 oriented normal to the coils 1-4.

The spatial configuration of the excitation field 220 in the excitationvolume 209 can be quickly adjusted by adjusting the switch network 206(FIG. 10) and changing the phases of the electrical signalsindependently provided to the coplanar coils 1-4. As a result, theoverall magnetic excitation field can be changed to be substantiallyoriented in either the X, Y or Z directions within the excitation volume209. This adjustment of the spatial orientation of the excitation field220 effectively avoids any blind spots in the excitation volume 209.Therefore, the remote, miniature leadless markers 14 within theexcitation volume 209 can be energized via the coplanar source coils 46regardless of the spatial orientations of the leadless markers.

In one embodiment, the source generator 18 is coupled to the sensorarray 16 so that the switch network 206 (FIG. 10) adjusts orientation ofthe pulsed generation of the excitation field 220 along the X, Y, or Zaxis, depending upon the strength of the signal received by the sensorarray. If an insufficient marker signal is received from the sensorarray 16, the switch network 206 can be automatically adjusted to changethe spatial orientation of the excitation field 220 during a subsequentpulsing of the source coils 46 to generate the excitation field orientedalong a different axis or oriented between the axes. The switch network206 can be adjusted until the sensor array 16 receives a sufficientsignal, thereby ensuring sufficient excitation of the leadless markers14 within the excitation volume 209.

Although specific embodiments of, and examples for, the presentinvention are described herein for illustrative purposes, variousequivalent modifications can be made without departing from the spiritand scope of the invention, as will be recognized by those skilled inthe relevant art. The teachings provided herein of the present inventioncan be applied to systems for excitation of leadless miniature markers,not necessarily the exemplary system generally described above.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A system for generating a spatially adjustable excitation field forexcitation of a remote leadless marker assembly, comprising: a sourcegenerator that generates a plurality of, alternating electrical signalseach having an independently adjustable phase; and a plurality ofexcitation coils being configured to simultaneously receive a respectiveone of the alternating electrical signals at a selected phase togenerate a magnetic field, the phase of the alternating electricalcurrent for each excitation coil being independently adjustable relativeto the phase of the alternating electrical signal for the other ones ofthe excitation coils to adjust the magnetic field from the respectivecoil; the magnetic fields from the excitation coils being combined toform a spatially adjustable excitation field for excitation of theremote leadless marker assembly.
 2. The system of claim 1 wherein theexcitation coils are co-planar.
 3. The system of claim 1 wherein thecoil axes of at least two excitation coils are parallel.
 4. The systemof claim 1 wherein the coil axes of at least two excitation coils arenon-concentric.
 5. The system of claim 1 wherein the plurality ofexcitation coils includes four co-planar excitation coils.
 6. The systemof claim 1 wherein the alternating electrical signal for each coil isnon-sinusoidal.
 7. The system of claim 1 wherein the plurality ofexcitation coils include four side-by-side excitation coils each havingstraight sides parallel to at least one side of two other of theexcitation coils.
 8. The system of claim 1 wherein the plurality ofexcitation coils includes at least two co-planar excitation coils. 9.The system of claim 1 wherein the excitation coils are mounted on asubstantially planar substrate.
 10. The system of claim 1 wherein theexcitation coils include first and second excitation coils, the firstexcitation coil being configured to receive an alternating electricalsignal with a first selected phase to generate a first magnetic field,and the second excitation coil being configured to receive analternating electrical signal with the first selected phase to generatea second magnetic field to combine with the first magnetic field togenerate a first shaped excitation filed, and the second excitation coilbeing configured to receive an alternating electrical signal with asecond selected phase substantially different from the first selectedphase to form a third magnetic field that combines with the firstmagnetic field to form a second excitation field with different spatialcharacteristics than the first excitation field.
 11. A system forgenerating a spatially adjustable excitation field for excitation of aleadless marker assembly, the system being connectable to a sourcegenerator that generates a plurality of alternating electrical signalshaving independently adjustable phases, the system comprising: aleadless marker assembly; and a plurality of excitation coils remotefrom the leadless marker assembly, each excitation coil beingindependently coupled to the source generator; the excitation coilsbeing configured to simultaneously receive a respective one of thealternating electrical signals at a selected phase to generate amagnetic field, the phase of the alternating electrical signal for eachexcitation coil being independently adjustable relative to the phase ofthe alternating electrical signal for the other ones of the excitationcoils to adjust the magnetic field from the respective coil; themagnetic fields from the excitation coils being combined to form aspatially adjustable excitation field for excitation of the remoteleadless marker assembly.
 12. The system of claim 11 wherein theexcitation coils are co-planar.
 13. The system of claim 11 wherein theplurality of excitation coils includes four co-planar excitation coils.14. The system of claim 11 wherein the coil axes of at least twoexcitation coils are parallel.
 15. The system of claim 11 wherein thecoil axes of at least two excitation coils are non-concentric.
 16. Thesystem of claim 11 wherein the plurality of excitation coils includes atleast two co-planar excitation coils.
 17. The system of claim 11 whereinthe excitation coils are mounted on a substantially planar substrate.18. The system of claim 11 wherein the excitation coils include firstand second excitation coils, the first excitation coil being configuredto receive an alternating electrical signal with a first selected phaseto generate a first magnetic field, and the second excitation coil beingconfigured to receive an alternating electrical signal with the firstselected phase to generate a second magnetic field to combine with thefirst magnetic field to generate a first shaped excitation filed, andthe second excitation coil being configured to receive an alternatingelectrical signal with a second selected phase substantially differentfrom the first selected phase to form a third magnetic field thatcombines with the first magnetic field to form a second excitation fieldwith different spatial characteristics than the first excitation field.19. A system for generating an excitation field for excitation of aleadless marker assembly, the system being connectable to a sourcegenerator that generates a plurality of alternating electrical signalshaving an independently adjustable phase, the system comprising: aplurality of excitation coils configured to generate a magnetic fieldfor excitation of the leadless marker assembly; and a switching networkcouplable to the source generator, the switch network having a pluralityof switches independently coupled to the excitation coils, the pluralityof switches being configured to selectively direct electrical signalsthrough respective ones of the excitation coils to independentlygenerate an alternating magnetic field with a selected phase for eachexcitation coil; and the magnetic fields from the excitation coils beingcombined to form a spatially adjustable excitation field for excitationof the remote leadless marker assembly.
 20. The system of claim 19wherein the excitation coils are co-planar. 21-48. (canceled)